The invention relates generally to an imaging system utilizing imaging sheets having a surface coating of rupturable photosensitive microcapsules and, more particularly, to apparatus for rupturing the microcapsules on such imaging sheets.
U.S. Pat. Nos. 4,440,846 and 4,399,209, which are assigned to the same assignee as this invention, and which are herein incorporated by reference, describe an imaging system wherein a photosensitive layer, comprising microcapsules containing a photosensitive composition in the internal phase, is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. The imaging system is particularly advantageous because it is a totally dry system and does not rely upon the application of wet developing processing solutions to produce the image. An image-forming chromogenic material, such as a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former image-wise reacts with the developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing image-wise exposed imaging sheets through the nip between a pair of parallel calender rolls.
The media may exist in either single-sheet or two-sheet versions. In the former case, the microcapsules and developer composition are both coated onto a single substrate layer. In the latter case, the microcapsules are carried on a first substrate layer referred to as a donor sheet. The developer composition is coated onto a second, separate substrate layer referred to as a receiver sheet. The donor sheet is subjected to the actinic radiation, and the exposed microcapsule layer is then brought into contact with the developer layer of the receiver sheet. The two sheets are then developed by pressure, with the finished image being formed in the receiver sheet.
While heavy pressure is not required to rupture the microcapsules, high pressure and large calender rolls are normally used to develop the imaging sheets. To normalize surface irregularities in the imaging sheets, substantial compression of the paper must be achieved. Thus, the use of large diameter rolls has been considered necessary to achieve a uniform distribution of the rupturing force across the surface of the imaging sheets. Otherwise, if the rupturing force is not uniformly distributed, the imaging sheets develop unevenly and the tonal characteristics of the resulting images are degraded.
One significant complication in applying apparatus for fixing fusible toner images to the development of microencapsulated media sheets is the relative magnitudes of pressure required. Typical pressure roll devices for fixing fusible toner images can apply approximately 1000 lbs/in.sub.2. For the microencapsulated media, pressures of 6000 to 8000 lbs/in.sub.2 are required.
In pressure fixation apparatus for fixing a dry pressure fusible toner image onto a sheet of paper, it is known to use a pair of pinch rolls biased toward each other and disposed such that the axis of one roll extends at an angle relative to the axis of the other. This skewed arrangement tends to minimize irregularitres of the nip between the rolls even when a paper sheet passes therebetween. However, when the skew angle is too large, creases or wrinkles are formed in the paper sheet. In order to avoid such wrinkles in the sheet, the skew angle is kept quite small. With a small skew angle, however, the rolls again must have large diameters to ensure uniform pressure along the nip due to the bending of the rolls. In addition to the added weight of the larger rolls, the loading pressure must also be increased. Hence, larger pressure mechanisms, higher compression resistant bearings, housings and so forth lead to an increased cost for the apparatus.
To overcome these difficulties in the art of fixing fusible toner images, three roll systems have been used. Such a system is shown in U.S. Pat. No. 4,343,234. In one three roll system, the two upper rolls form a pair of pinch rolls which are disposed parallel and horizontal with a slight skew angle therebetween. A third back-up roll is disposed in a crossing contact relation to the second roll. The back-up roll is biased resiliently upwards toward the second roll of the pair of pinch rolls by a pressure mechanism such that the second roll is butted against the first roll under pressure to form a nip along their contacting line through which paper sheets pass. The bending of the second roll can be adjusted by the cross angle of the back-up roll relative to the second roll such that both the second roll and the back-up roll may have a smaller diameter than the first roll. Even in this case, however, the skew angle between the nip rolls must still be relatively small to avoid wrinkling of the sheets.
A particular inherent disadvantage of such prior art fixing or developing systems resides in the fact that the pressure across the nip is uniform only at one design pressure, and variations in the thickness of the material passing through the nip or variations in the pressure applied to the nip, causes a departure from a condition of linearity across the nip.
Improved pressure development apparatus for imaging sheets employing photosensitive microcapsules, as described above, are disclosed in co-pending application Ser. No. 009,851 filed Feb. 2, 1987, now U.S. Pat. No. 4,727,392, incorporated herein by reference. In the co-pending application, the problems introduced by skew angles at the nip or by differential roll speeds at the nip caused by crowned rolls are eliminated by the employment of separate crown rolls which coact with the pressure rolls. This allows the pressure rolls to be cylindrical and free of skew angle and free of differential surface speeds at the nip. Also, improved sheet feed arrangements are disclosed by which the sheet material may advantageously be applied to the pressure rolls in such a manner as to reduce the tendency for wrinkling of the imaging or donor sheets.
In spite of the improvements which have been made to high pressure roll development apparatus, there exists a need for apparatus which is lighter in weight, which is capable of providing a substantially uniform nip pressure throughout a design width of the roll over varying thicknesses of sheet material and over varying overall loadings on the rolls, and to provide a roll-type pressure developer which is inherently capable of applying a uniform pressure across a nip, over long periods of time, with a minimum of maintenance requirements.